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"Office workers must exercise for an hour a day to counter death risk," The Daily Telegraph reports.

A major new study suggests that at least an hour's exercise a day may compensate for the risks of a sedentary lifestyle.

The study, which looked at previous research involving more than a million people, delivered a "bad news, good news" analysis. The bad news is that sitting for long periods may increase the chance of dying earlier. The good news is that doing at least an hour of moderately intense activity (such as cycling or brisk walking) each day may eliminate that risk.

The people in the study who were least active and sat for more than eight hours a day were 59% more likely to have died during the study follow-up than people who exercised most and sat for less than four hours a day. Sitting for longer than four hours a day increased the chance of death for everyone not in the highest activity category. However, people who did the most physical activity did not have an increased risk of death, regardless of how many hours a day they spent sitting.

This type of research cannot prove cause and effect but it certainly seems that daily physical activity brings long-term benefits.

The current activity advice for adults is to do at least 30 minutes of physical activity a day. Increasing that to 60 minutes may be a good idea if you do have a "9-5 office lifestyle".

Where did the story come from?

The study was carried out by researchers from institutions in many different countries, including the Norwegian School of Sports Sciences, University of Cambridge, University of Queensland, Oslo University Hospital, Swinburne University of Technology in Melbourne, Sydney University and Harvard Medical School. It received no direct funding.

Some UK media outlets took the study very literally. The Daily Mail tells readers "adults who sit down for at least eight hours every day must do at least an hour's daily exercise to undo all the harm." The study does not prove that exercise will "undo the harm" of sedentary behaviour.

It also ignores the study findings that people who were moderately active for about half an hour to an hour had only a slightly raised risk of death associated with sitting for longer periods. While the advice to exercise more is sound, people might think there's no point in exercising for less than an hour a day, and so give up altogether. It is very much the case that "every little helps" when it comes to exercise.

Experts in sports and exercise medicine were mostly welcoming of the study, describing it as "excellent quality" and "very interesting". However, one expert in evidence based medicine warned of the study's limitations and that it had not sufficiently controlled for factors such as socioeconomic status.

What kind of research was this?

This study was a systematic review and meta-analysis of prospective cohort studies. The researchers went back to the authors of the studies and asked them to re-analyse their data according to a standardised protocol, which allowed them to make direct comparisons between groups.

This is a good way to get a better idea of the relative importance of sitting and physical activity in terms of length of life. However, observational studies cannot tell us whether certain factors (sitting time or physical activity) directly cause another (chances of death). They can only tell us that the factors may be linked.

What did the research involve?

Researchers searched the literature for studies that included information on sitting time, exercise and mortality. They added two studies that had not been published but which had relevant data.

They asked the original study authors to rework their data according to a standardised protocol which divided people into categories of physical activity and sitting time. They then pooled the data to look at how the two factors were linked to length of life. They also looked separately at time spent watching television, and at deaths from cardiovascular disease and cancer.

By applying a standardised protocol, the researchers were able to make direct comparisons across groups according to specific categories of sitting time (less than four hours a day, four to six hours, six to eight hours, and more than eight) and of physical activity. Physical activity was measured by metabolic equivalent of task (MET) hours a week. MET is a measurement of how much energy the body is likely to consume during specific physical activities.

MET levels were divided in four groups:

less than 2.5 (equivalent to five minutes a day of moderate intensity physical activity)

16 (25 to 35 minutes a day, as recommended by many guidelines)

30 (50 to 65 minutes a day)

more than 35.5 (60 to 75 minutes a day)

Researchers took the people who did the most physical activity and had the least sitting time as the baseline, and looked to see how more sitting time affected that, for people in the different categories of physical activity.

The same calculations were repeated using daily hours of TV viewing time, from less than one to five or more.

What were the basic results?

For people who did the least physical activity, sitting for more than four hours a day was linked to an increased chance of dying during the study. For these people, sitting for eight hours a day or more increased the chances of death by 27% (hazard ratio (HR) 1.27, 95% confidence interval (CI) 1.22 to 1.32), compared to if they'd been sitting four hours a day or less. Compared to people who did the most exercise and sat for less than four hours a day, they had a 59% increased risk of death (HR 1.59, 95% CI 1.52 to 1.66).

People who were physically active for between half an hour and an hour also had a raised chance of death linked to sitting for eight hours a day compared to four hours a day, of 10% to 12%. But for people who exercised the most, time spent sitting did not increase the risk of death.

High levels of physical activity were clearly linked to lower chance of death. People who did the most activity but sat for eight hours or more were less likely to die than those who did the least activity but sat for four hours or less.

Television viewing time showed similar results, but in this case even the highest amount of physical activity did not cancel the raised risk of watching five hours or more of television. The least active people had a 44% higher risk of death if they watched five or more hours of television, compared to less than one hour (HR 1.44, 95% CI 1.34 to 1.56).

Results were similar when the researchers looked at the chances of dying from cardiovascular disease or cancer.

How did the researchers interpret the results?

The researchers concluded: "These results provide further evidence on the benefits of physical activity, particularly in societies where increasing numbers of people have to sit for long hours for work" and suggest the study should be taken into account when public health recommendations are made.

Conclusion

This study helps to disentangle the effects of having a sedentary lifestyle and being physically active. Previous studies have had conflicting results, with some saying that sitting for long periods can be counteracted by taking exercise, while others disagreed.

The advantage of this study is that it looks at time spent sitting as well as time spent being physically active, and calculates how both are linked to mortality and to each other.

The study has many strengths, not least its size. It includes data from 1,005,791 people from 16 studies. The researchers applied a standardised protocol and asked study authors to provide re-analysed data. This meant they could pool information and make direct comparisons between groups sub-divided by sitting time and activity levels, to a higher degree of accuracy than would otherwise have been possible.

However, there are limitations. The authors only included English-language papers, so other relevant studies may have been excluded.

The authors tried to account for what is called reverse causation – in this case that illness may have prevented people from being physically active – by including studies of apparently healthy adults. However they admit that this factor was not completely ruled out.

In addition, the data came from participants' own estimates of time spent sitting, watching TV and being physically active. Not only is this reliant on accurate (and honest) self-assessment, it was only measured at one time point, so may not be representative over time.

Although the original studies included controls for most other confounding factors, such as smoking, most did not include socio-economic data, which could have a big impact on the results. For example, watching a lot of television could be linked to being on a low income, or unemployed, which are themselves linked to poor health.

Conversely, going to the gym is expensive, so this type of physical activity may be more common among people who are better-off. That makes it hard to know whether TV watching or exercise are the factor causing the difference in death rates, rather than being a marker for something else.

We know that sedentary lifestyles are linked to poorer health. For many people, work (or travelling to work) involves sitting down for long periods. While some people may be able to change this, for example by using a standing desk or cycling to work, for others it's not so easy. So it's heartening to know that taking exercise and being physically active in your free time may help.

However, it's interesting to note that the levels of activity linked to eliminating the risk of a sedentary lifestyle are higher than those usually recommended. The most active people spent the equivalent of 60 to 75 minutes doing moderately intense physical activity – higher than the usually recommended 30 minutes a day.

It may be that compensating for a desk job requires us to be more physically active than most of us currently manage.